1. Technical Field
The present invention relates generally to paving equipment. More particularly, the present invention relates to a paving material density indicator and method for determining density of paving material by providing a sensor with a shape and size configured to have an electrical field with at least one of a controlled depth of penetration, controlled area, controlled shape, and controlled volume.
2. Related Art
During paving, paving material is usually laid at about 75% of acceptable compaction. However, during subsequent compaction it is highly advantageous to assure that the paving material is compacted to as close to total acceptable compaction as possible. Attaining total acceptable compaction assures the lack of variations in the material, such as air voids, that can create potential defects in the paving material. Unfortunately, the level of compaction is not readily apparent to the naked eye. In order to address this problem, measurement of dielectric properties of paving material is known to be very useful for determining material density, a key indicator of compaction level.
However, due to pavement laying machines used today becoming more adept at laying thinner pavement lifts, it has become necessary to have a reliable and convenient density determining device capable of measurement at differing depths, areas, shapes, and volumes of the pavement. Additionally, it has become necessary to have a density determining device which is faster than those available. Heretofore, no adequate paving material sensor capable of addressing the above problems has been devised. Accordingly, the present invention addresses a long felt need in the art.
One pavement density indicator device is that of Blackwell, U.S. Pat. No. 3,784,905. Blackwell's device measures dielectric properties of the asphalt which is representative o the change in density in the asphalt. The device of Blackwell has many disadvantages. For example, in order to obtain a reading, the Blackwell device must be moved at extremely slow speeds across the material being tested and, accordingly, requires an extended time period to provide a determination. The Blackwell device, due to its excessive weight, also requires a large sled frame (contact area) to be dragged across the pavement surface. Another disadvantage is limited adjustability of the depth of measurement of the device caused by the given set of electrodes only being able to vary the depth of measurement by changing the height of the electrodes.
In another apparatus, a nuclear source is used to determine density of pavement material. This device has a variety of obvious drawbacks. For instance, the device requires a licensed operator and a radiation shield (e.g., a lead enclosure). Furthermore, the device is non-adjustable for area and depth, time consuming in use, and heavy. The device is also very expensive.
Another disadvantage of the related art devices lies in their inability to adjust depth of measuring for variations in depth of pavement (e.g., when a new layer of pavement, called a lift, has been laid). When the related art devices are used to determine the density of a new layer of pavement, the measurement may go beyond the layer of new pavement and into the material below, hence, providing an inaccurate density indication. Similarly, the related art devices may not sense far enough into the new layer, e.g., where a dip has been filled, to assure a constant compaction level within the pavement lift.
Another disadvantage of the related art is their inability to vary the shape and area of the sensing area. Altering the shape and area of the sensor is advantageous for determining the density in particular pavement attributes, e.g., dips, joints odd shaped patches, etc.
Other devices, such as that of Harris, U.S. Pat. No. 3,400,331, incorporated herein by reference, have been used to detect the presence and dimensions of an object. However, Harris is incapable of detecting the density of the object. Furthermore, the Harris device requires that the sensor head remain as small as possible, a disadvantage to unlimited size and shape measurement.